The objective of this project is to characterize dose-response relationships and toxicity of tetrafluoroethylene administered by inhalation to rats and mice in 2-year chronic studies. Animals will be exposed for 6 hours per day, 5 days per week for 2 years. At the end of the study, all animals will be given gross and microscopic histopathologic evaluations. A 65-week interim sacrifice, including hematology and histopathology will be performed in the chronic study. The results of these studies will be used by NTP to assess the toxicologic and carcinogenic potential of exposure to tetrafluoroethylene.